Mobo & RAM: two sticks individually - good, two sticks together - bad

I am trying to get my two sticks (2x 2Gb) of Corsair RAM (CM2X2048-6400C5C) to work in my Asus P5Q Deluxe mobo.
Both sticks work perfectly because I have tested them in all slots individually and the computer works flawlessly.
However, the problem that I'm having is when I put both sticks in, be that in A1/B1 or A2/B2.

With both sticks in, the computers powers on, the fans spin up but the computer progresses no further and does not POST - a blank screen.